I was worrying a lot about my Chemistry last year too. In the end I managed to stop worrying by thinking that there's nothing you can do to change the outcome, so why worry? Easier said than done, I know, but you have to try. You might have done better than you anticipate and are worrying for no reason! Maybe it helps if you assess what are the possible outcomes of results day and make a plan for each outcome - there is always something you can do to get into Medicine. It might mean more effort, less chances of success and many complications, but there are several routes into Medicine and really, it's only over whenever you call it over.
Good luck!